net_sched: sch_fq: handle non connected flows

FQ packet scheduler assumed that packets could be classified
based on their owning socket.

This means that if a UDP server uses one UDP socket to send
packets to different destinations, packets all land
in one FQ flow.

This is unfair, since each TCP flow has a unique bucket, meaning
that in case of pressure (fully utilised uplink), TCP flows
have more share of the bandwidth.

If we instead detect unconnected sockets, we can use a stochastic
hash based on the 4-tuple hash.

This also means a QUIC server using one UDP socket will properly
spread the outgoing packets to different buckets, and in-kernel
pacing based on EDT model will no longer risk having big rb-tree on
one flow.

Note that UDP application might provide the skb->hash in an
ancillary message at sendmsg() time to avoid the cost of a dissection
in fq packet scheduler.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>